He has compiled a vocabulary (6)
I believe the answer is:
alexis
'he' is the definition.
'compiled a vocabulary' is the wordplay.
I cannot quite see how this works, but
'a' is found in the answer.
The remaining letters 'lexis' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'has' acts as a link.
Can you help me to learn more?
(Other definitions for alexis that I've seen before include "Lia's ex has another name" , "man or woman" .)